In an image forming apparatus having a plurality of image forming units, driving unevenness occurs in the device due to factors such as lack of machine accuracy or the like, causing a misregistration (color deviation) in each color. Particularly in an apparatus having an image forming unit including a laser scanner and a photosensitive drum for each color, if a distance between the laser scanner and the photosensitive drum differs in the image forming units of respective colors, a difference is generated in a laser scanning width on the photosensitive drum, resulting in a color deviation.
In view of this, there is a technique for making various adjustments to correct the misregistration. That is, misregistration detection patterns are formed on a conveying belt, then the positions of the misregistration detection patterns are detected by an optical sensor, and the misregistration is corrected in accordance with the detected amount of misregistration.
An example of misregistration is shown in FIG. 1. Numeral 100 denotes an original image position; and 110, an image position where a misregistration is generated. Note that although numerals 110a, 110b, and 110c show cases where there are misregistrations in the scanning direction, the two lines are drawn apart in the conveying direction for description purposes.
Numeral 110a denotes a gradient gap of a scanning line, which is generated in a case where there is a gradient between a photosensitive drum and an optical unit such as a laser scanner. The gradient gap can be corrected in the arrow direction by, for instance, adjusting a position of the lens or a position of the photosensitive drum and the optical unit.
Numeral 110b denotes a misregistration generated by uneven scanning widths, which is caused by a different distance between the optical unit and the photosensitive drum or the like. It is often generated in a case where the optical unit is a laser scanner. The misregistration can be corrected in the arrow direction by, for instance, slightly adjusting the image frequency (if the scanning width is long, the frequency is raised) and changing the length of the scanning line.
Numeral 110c denotes a write-start position error in the scanning direction. Assuming that the optical unit is a laser scanner, the write-start position error can be corrected in the arrow direction by, for instance, adjusting the write-start timing at the beam detection position.
Numeral 110d denotes a write-start position error in the printing paper conveying direction. The write-start position error can be corrected in the arrow direction by, for instance, adjusting the write-start timing of each color upon detection of a printing paper edge.
Assume herein that misregistration detection patterns for each color of yellow (Y), magenta (M), cyan (C), and black (K) are formed on the conveying belt. The positions of the patterns are detected by a pair of optical sensors provided on both sides of the conveying belt on the downstream unit, and various adjustments are made to correct the misregistration in accordance with the detected amount of gap.
However, in detection of the misregistration detection patterns, the result is influenced by uneven driving of the photosensitive drum and uneven driving of the conveying belt driving rollers. In view of this, for instance, Japanese Patent Applications Laid-Open No. 2001-356542 and No. 2002-23445 disclose the technique for arranging the misregistration detection patterns in a way that unevenness in the cycles of the conveying belt driving rollers is averaged and cancelled.
However, in this technique of arranging the misregistration detection patterns so as to cancel the unevenness in the cycles of the photosensitive drum and the unevenness in the cycles of the conveying belt driving rollers, it is necessary to arrange the misregistration detection patterns within one periphery of the conveying belt. If the misregistration detection patterns cannot be arranged within one periphery of the conveying belt, it is necessary to perform cleaning using means to collect toner on the conveying belt in the cartridge before all the misregistration detection patterns are formed, thus requiring increased downtime.
Particularly in a small image forming apparatus, since the peripheral length of the conveying belt is short, it is difficult to arrange the conventional misregistration detection patterns within one periphery of the conveying belt. Although it may be possible to arrange the misregistration detection patterns at high density within one periphery of the conveying belt by employing special sensors having a small spot diameter, it causes an increased cost.